1. Field of the Invention
The present invention relates generally to liquid crystal displays and more specifically to a method and apparatus of multiplexing liquid crystal displays.
2. Description of the Prior Art
The use of low power liquid crystal display elements in, for example, portable calculators, watches, etc., is desirable since these environments generally use batteries and consequently the life of a battery is extended. Similarly, the circuit elements needed to drive the liquid crystal display reduces the overall size of the integrated circuit. One of the problems encountered with a liquid crystal display, versus light emitting diodes, is that the liquid crystal display is a bidirectional element and consequently unselected segments may be activated for a given digit because of the bidirectionality of the liquid crystal device.
Liquid crystal devices are generally on for a voltage differential of V1 across it and are completely off for a voltage differential below a voltage V2 across it. In order for the segment to be completely off, it must have less than V2 across it. If the segment data input changes from zero for segment off to V1 for segment on, extraneous paths can be set up from the V1 of an on segment to the zero of an off segment of the same digit and consequently activate the nonselected digit. Other multiplexing schemes which multiplex the digits and the segments by varying the voltage on the digit input and the segment input between zero and V1 in the opposite direction so as to produce a voltage V1 thereacross, have the same problem. Namely, the extraneous paths are set up such that unselected elements are activated. For other multiplexed displays having a plurality of digits with the corresponding segments connected to a common drive, the difference between the segment non-drive voltage and the digit non-select voltage is not sufficiently low to turn off a segment activated in a previously selected digit and also activated in the next digit.
It has also been noted as a problem by the prior art that DC multiplexing schemes of AC-type liquid crystal displays shorten the life of the display element because of chemical plating action which occurs. Prior art attempts at solving this problem have included complicated current reversal circuits on either the digit or segment drive. Similarly, circuits involving reversal circuit in both the segment and digit input have not recognized the problem discussed in the previous paragraph concerning the extraneous path set up by using a reversal of zero and V1 voltage levels.
Thus there exists a need to provide a multiplexing scheme which will end the extraneous conduction paths for liquid crystal displays and a polarity reversal scheme which will prevent plating in AC liquid crystal displays and also prevent the formation of the extraneous current path.